Bluetooth supports two types of transports: synchronous connection oriented (SCO) transports and asynchronous connectionless link (ACL) transports. Bluetooth profiles that require a full-duplex, low latency voice link, such as the Hands-Free Profile (HFP), require an SCO transport between an audio gateway (AG), such as a mobile phone, and a Hands-Free Unit (IF), such as a simple headset or a hands-free unit installed in a car. (Other profiles that require a full-duplex voice link include the Headset Profile (HSP), the Cordless Telephony Profile (CTP), and the Intercom Profile (ICP).)
The HFP uses the SCO transport and a Continuously Variable Slope Delta Modulation (CVSD) codec to stream full-duplex narrow bandwidth (8 kHz sampling rate) voice between the AG and HF. The SCO transport also requires periodic allocation of a pair of slots once every 2, 4, or 6 slots. The SCO transport does not include a retransmission mechanism and is based on a fixed scheduling. Thus, when in the presence of an interfering synchronous signal (e.g., LTE, WiMAX, or WCDMA operating at the 2.5 GHz band while Bluetooth is operating at the 2.4 GHz band), packet losses over SCO links due to adjacent channel interference and receiver de-sense may be as high as 25% on the downlink (AG to HF) and 38% on the uplink (HF to AG). Enhanced SCO (eSCO) enables limited retransmissions and a wider variety of packet types, but eSCO has the same fixed scheduling as SCO which can be extremely detrimental in the presence of certain types of interference.
The other type of Bluetooth transport, ACL, is more reliable in interference conditions due to a retransmission mechanism. ACL, unlike SCO/eSCO, uses non-fixed scheduling. The Audio/Video Distribution Transport Protocol (AVDTP) is a transport protocol based on the ACL transport, and it is used by the Advanced Audio Distribution Profile (A2DP) of Bluetooth. A2DP uses AVDTP and a Sub-Band Codec (SBC) or other supported codec to stream wide bandwidth (16 kHz or higher sampling rate) audio in one direction only—from a source (SRC) to a sink (SNK). Another type of ACL transport protocol is Bluetooth Network Encapsulation Protocol (BNEP), which is required by the Personal Area Networking Profile (PAN) of Bluetooth. BNEP enables point-to-multipoint IP-based communication over an ACL transport.
There is an opportunity to establish a Voice over ACL (VoACL) link for streaming full-duplex wide bandwidth voice using Bluetooth. This would enable the use of a wide range of frame-based codecs such as an Adaptive Multi Rate-Wideband (AMR-WB) codec, an internet Low Bitrate Codec (iLBC), a Speex codec, and others. SCO links only support Continuously Variable Slope Delta Modulation (CVSD) and Pulse Code Modulation (PCM) codecs, which require a high bandwidth although these codecs only provide mediocre audio quality. Establishing a Bluetooth VoACL transport may also help mitigate some negative effects of coexistence of Bluetooth with technologies such as LTE, WiMAX, and WCDMA. Flexibility in scheduling of ACL traffic enables a controller to try to minimize collisions by scheduling Bluetooth packets in-between the transmissions of interfering technologies. Additionally, the retransmission mechanisms existing in Bluetooth ACLs may reduce packet losses. The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Drawings and accompanying Detailed Description.